The present invention relates to a technology to adopt a sufficient measure for safety by assuring the supply voltage to the control circuit even when the supply of the power supply voltage to the discharge lamp or the lighting circuit is cut off because of the operation of the overcurrent protecting means.
As the lighting circuit for the discharge lamp (the metal halide lamp, etc.), the configuration having the DC power supply circuit, the DC-AC converter circuit, and the starting circuit (so-called starter circuit) is well known.
FIG. 8 shows a circuit configuration 50 of the discharge lamp lighting circuit according to the prior art. The power is supplied from the DC power supply 52 to the lighting circuit 51 via the overcurrent protecting element c and the lighting switch 55.
In order to control the power supply to the discharge lamp f connected to the output terminal of the lighting circuit 51, the control circuit 57 is provided. As the power supply voltage supplied to the circuit, the DC voltage that is obtained from the DC power supply 52 via the overcurrent protecting element 53 and the lighting switch d may be employed as it is, or may be generated by the power supply circuit 54 based on the DC voltage.
By the way, in the event that the lighting of the discharge lamp become failure or the input voltage to the lighting circuit has the abnormal value, if the power supply voltage enough to operate the control circuit can be always assured, the injury upon the human body because of the high voltage can be prevented or bad effects such as the smoke the ignition, etc, due to the excessive output of the power can be prevented previously since the operation of the lighting circuit can be stopped at the time of failure.
However, in the circuit of the prior art, if the supply of the power from the DC power supply 52 is cut off because of the operation of the overcurrent protecting element 53, the supply of the power to the control circuit 57 is also stopped. Therefore, there is the problem that it is also impossible to adopt a sufficient measure for safety (e.g., lighting of the alternative light source, information of the failure occurrence, etc.).
Therefore, it is an object of the present invention to adopt a sufficient measure for safety by assuring the supply voltage to the control system circuit even when the supply of the power supply voltage to the discharge lamp or the lighting circuit is cut off due to the operation of the overcurrent protecting means.
In order to overcome the above subject, in the present invention, there is provided a discharge lamp lighting circuit comprising a DC power supply portion for receiving a supply voltage from a DC power supply via an overcurrent protecting means as an input voltage and converting the input voltage into a desired DC voltage; a DC-AC converter portion for converting an output voltage of the DC power supply portion into an AC voltage to supply the AC voltage to a discharge lamp; a starter circuit for starting the discharge lamp by applying a high voltage pulse to it; and a control circuit for detecting a voltage applied to the discharge lamp or a current flowing through the discharge lamp or detecting further the input voltage to control a supply power to the discharge lamp or to decide whether or not a failure occurs in a lighting state of the discharge lamp or in the input voltage; wherein a voltage obtained from a position that is located closer to the DC power supply than the overcurrent protecting means or a voltage generated based on such voltage, or a voltage supplied from another system different from the DC power supply or a voltage generated based on such voltage is supplied to the control circuit as the power supply voltage.
Therefore, according to the present invention, the situation that the power cannot be supplied to the discharge lamp because of the operation of the overcurrent protecting means is brought about, the power supply voltage to the control circuit can be obtained by the voltage obtained from the position that is located closer to the DC power supply than the overcurrent protecting means or the voltage supplied from another system different from the DC power supply.